IV. The Stomatal Apparatus: The Path of the Gas Supply in Plants

Abstract

In that remote age when the ancestors of our present day plants left the aquatic environment and became adapted to land conditions, they had to undergo severe changes. Whereas they had been accustomed to being surrounded by water, they had, under the new conditions, to resist the drying effects of the atmosphere. If they had developed a water-proof skin the difficulty would have been solved: but a water-proof skin is also a gas-proof skin; and a plant, like an animal, needs to have gaseous oxygen to enable it to respire ; unlike an animal, it also needs gaseous carbon dioxide (which it takes in and makes into starch, cellulose, sugars, proteins and most of those substances which constitute the plant body). Thus a gas-proof skin would mean that the plant would be unable to get the gas necessary for its food or the gas necessary for its energy. The plants were virtually faced with two kinds of death - on the one hand by drying up, on the other hand by starving to death through lack of gas. The situation was saved by a compromise, and the result was the delicate complicated structure which is visible in a leaf under the microscope. This resultant development of leaf structure was by no means sudden, and it has taken many, many years for all the diverse leaf forms to evolve.